Cyclic adsorption process using centrifugal compressors

ABSTRACT

A cyclic adsorption process is provided, the process containing one or more adsorber vessels undergoing the steps of at least pressurization and depressurization and driven by one or more variable speed centrifugal machines operating under acceleration and deceleration conditions and adjusted to the steps, vessel size, and process conditions employed, wherein the process cycle time is greater than the ratio of the change in inertia, defined the maximum energy that can be lost during a cycle due to inertia changes, to 0.3 times the total power of the one of more centrifugal machines that would be consumed in the absence of inertial effects.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/368,403, filed on Feb. 8, 2012, the disclosure of which isincorporated by reference herein.

FIELD OF THE INVENTION

The present invention provides an improved cyclic adsorption system andprocess for separating components of a gas stream using centrifugalmachines. More particularly, the present invention is directed toadsorption processes using one or more variable speed driven centrifugalcompressors operating under cyclic acceleration and decelerationconditions wherein the process cycle time is selected to minimize thecompressor power consumption and maximize the efficiency of the process.

BACKGROUND OF THE INVENTION

Cyclic adsorption processes are well know and are typically used toseparate a more absorbable component gas from a less absorbablecomponent gas. Examples include pressure swing adsorption (PSA) orvacuum pressure swing adsorption (VPSA) processes which use a lowpressure or a vacuum and a purge gas to regenerate the sorbent andtemperature swing adsorption (TSA) processes which uses a thermaldriving force such as a heated purge gas to desorb the impurities. Suchprocesses are generally used to separate oxygen or nitrogen from air,other impurities like hydrocarbons and/or water vapor from feed airgases, hydrogen from carbon monoxide, carbon oxides from other gasmixtures, and the like. These processes are also used to removeimpurities such as water vapor and hydrocarbons from air prior tocryogenic air separation. Any cyclic adsorption system for separating orpurifying gas components can be used.

For illustrative purposes, a typical VPSA process for separating oxygenfrom air is described herein although the present invention can beemployed with other cyclic adsorption processes using centrifugalcompressors and is not intended to be limited to this process. Thetypical cyclic VPSA process is one wherein an adsorber bed undergoes thefollowing stages:

1. The adsorber bed is pressurized to a desired pressure whereinnitrogen is readily adsorbed by the adsorbent as the feed air is passedacross the bed;

2. Product gas rich in oxygen is produced as the nitrogen in the feedair is adsorbed;

3. The bed containing the adsorbent is evacuated to a low pressure(typically under vacuum) wherein the adsorbed nitrogen is desorbed fromthe adsorbent in the adsorber bed; and, preferably,

4. A purge gas is passed through the bed to remove any remainingnitrogen.

The cycle time is understood by the skilled person to mean the amount oftime needed to complete one cycle; e.g. the process steps in order andthen return to the starting condition.

Some adsorption processes will have more steps or multiple beds andoften use one or more blowers for each of the pressurization anddepressurization steps. If the VPSA plant contains two or more adsorbervessels, each vessel undergoes the above steps; however, the two vesselsare operated out of phase so that while one vessel is producing productthe other is being regenerated. Also, in a two bed process two blowersare typically used wherein one is dedicated to feeding gas to theadsorber vessels while the other dedicated to evacuating the adsorbervessels.

Regardless of whether a single vessel, two vessels, or even more vesselsare used, the pressures and flows within the process change quickly asthe process cycles from adsorption to desorption. Generally, thepressure of a vessel will change from a low pressure condition of at orbelow atmospheric, preferably below atmospheric , such as about 6 to 8psia, to a high pressure condition of above atmospheric, such as about19 to 24 psia, in a rapid periodic cycle time, such as less than oneminute. Some adsorption processes will require even wider spans ofpressures and/or vacuums in similar rapid cycle times.

Traditionally, VPSA plants use positive displacement machines such asrotary lobe type blowers operating at fixed speeds to move gas throughthe process. These machines are robust and generally do not experienceany significant operational problems as the pressures and flows changeand reverse. However, these machines have low power efficiency andconventional machines are only 60-65% efficient. About 35-40% of theenergy supplied to these machines is therefore wasted. Thus, it isclearly desirable to replace the traditional rotary lobe machine with amore efficient machine capable of meeting the rigorous requirements ofrapid cyclic conditions.

One such machine is a centrifugal compressor driven by a direct drivevariable, high speed permanent magnet motor or a variable, high speedinduction motor. Such compressors have a known efficiency ofapproximately 85%. The challenge involved in the use of such acompressor is that its performance is very sensitive to changes inpressure, such as the rapid pressure changes that occur during apressure swing adsorption process. Centrifugal compressors used in rapidcyclic processes like the adsorption process described herein are highlysusceptible to frequent adverse operating conditions or states known assurge and stonewall as are more fully described below. Such conditionscan result in both low power efficiency and damage or failure to thecompressor impeller and other compressor or system components and havetherefore been avoided in practice. Thus, it is necessary to manage theadverse conditions of surge and stonewall if one is to succeed inreplacing positive displacement machines with more efficient centrifugalmachines in cyclic adsorption processes.

Centrifugal compressors have been proposed for use in adsorptionprocesses. For example, U.S. Pat. No. 5,555,749 suggests the use ofcentrifugal compressors in adsorption systems during the exhaust portion(depressurization) of the cycle. U.S. Pat. No. 7,785,405B2 disclosessystems and processes for gas separation using high-speed permanentmagnet variable-speed motors to accelerate and decelerate centrifugalcompressors used in pressure swing adsorption (PSA) or vacuum pressureswing adsorption (VPSA) processes. These patents do not teach a processin which the optimal cycle time is selected to realize the powerbenefits from the use of such compressors.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an improved process, and system for theuse of centrifugal machines in cyclic adsorption processes. Thisinvention provides a method for realizing the power benefits of usingcentrifugal machines in place of conventional positive displacementmachines by minimizing the adverse condition of stonewall associatedwith the rapid decreases in head pressure associated with the suddenoutflow or inflow of gas into the vessel during certain steps of theprocess. Although centrifugal machines are known to be more efficientthan conventional positive displacement machines, the power advantagesare not realized when used in such cyclic processes operating underacceleration and deceleration conditions unless the process cycle timeis adjusted to be more than a predetermined value calculated based onthe moment of inertia of the centrifugal impeller and other movingparts. By adjusting the adsorption process to use longer cycle times,stonewall can be minimized and the efficiency of the centrifugalmachines can be more fully realized.

In other words and as more fully explained below, the energy used by themachine during operation is the energy needed to compress the processgas from suction pressure to discharge pressure plus the energy neededto increase the speed of the compressor impeller (including rotor andother moving parts) from a low operating speed to a high operating speedfor the period needed to accomplish the required process steps. It hasnow been discovered that for a given centrifugal machine operating underacceleration/deceleration conditions, the energy needed to overcome themoment of inertia will always be fixed. Thus, the ratio of inertialenergy per cycle time (or inertial power) relative to total productionrate is minimized to achieve optimal operation of the centrifugalmachine.

According to one embodiment of this invention, a cyclic adsorptionprocess is provided comprising one or more adsorber vessels undergoingthe steps of at least pressurization, depressurization, and purge in thecycle and driven by one or more variable speed centrifugal machinesoperating under acceleration and deceleration conditions adjusted to thesteps, vessel size, and the process conditions employed, the improvementcomprising operating the process such that the cycle time is greaterthan the ratio of the Δ_(inertia) to 0.3 times the total power of theone of more centrifugal machines that would be consumed in the absenceof inertial effects.

According to another embodiment of this invention, a method is providedfor improving the power efficiency of a cyclic adsorption process usinga positive displacement machine containing one or more adsorber vesselsundergoing the steps of at least pressurization and depressurizationcomprising replacing the positive displacement machine with a variablespeed centrifugal machine; operating the centrifugal machine underacceleration and deceleration conditions adjusted to the steps, adsorbervessel size, and process conditions employed; and adjusting the processcycle time to be greater than the ratio of the calculated Δ_(inertia) to0.3 times the centrifugal machine power consumption in the absence ofinertial effects.

In yet another embodiment of this invention, a pressure-swing adsorptionprocess is provided for cycling between a high pressure condition and alow pressure condition and wherein the cycle includes at leastpressurization and depressurization steps, the process comprisingcyclically operating at least one variable speed centrifugal compressorbeing in fluid communication with at least one adsorber vessel toaccelerate from a low operating speed to a high operating speed toobtain the high cycle pressure condition and to decelerate from highoperating speed to low operating speed to obtain the low cycle pressurecondition whereby the cycle time is adjusted such that the centrifugalcompressor is in a stonewall condition for a period of not more than 40percent of the cycle time of the process under the conditions employed.

In still another embodiment, a method of using at least one centrifugalcompressor for both the pressurization and depressurization steps of anadsorber vessel used in a pressure swing or vacuum swing adsorptionprocess is provided wherein the compressor is adjusted to the specificsteps, adsorber vessel size, and process conditions employed, the methodcomprising:

using a variable frequency drive to control the motor driving thecentrifugal compressor from a low operating speed of not more than 7000RPM to a high operating speed of greater than at least 13000 RPM;

operating the compressor cyclically to accelerate from the low operatingspeed to a high operating speed and decelerate from high operating speedto low operating speed as required by the process for a period of notless than the ratio of the Δ_(inertia) to 0.3 times the centrifugalcompressor and motor power consumption rating in the absence of inertialeffects;

and wherein the compressor is in a stonewall condition for not more than40 percent of the total cycle time of the process under the conditionsemployed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a centrifugal compressor map with curvesindicating the flow generated by the centrifugal machine for variousoperating speeds and pressure ratios across the machine.

FIG. 2 is a graph showing a map of compressor efficiency as a functionof operating speeds and pressure ratios across the machine.

FIG. 3 is a graph using data from a VPSA cycle using a centrifugalmachine.

DETAILED DESCRIPTION OF THE INVENTION

As described above, the present invention provides an improved systemand process for the use centrifugal compressors in cyclic adsorptionprocesses. This system and process uses centrifugal compressors drivenby variable, high speed motors to replace conventional positivedisplacement machines and more specifically rotary lobeblowers/compressors to minimize power consumption and maximize processefficiency. Further, the selection of cycle time minimizes theoccurrence of stonewall found to result from the use of such centrifugalcompressors in these rapid cyclic adsorption processes.

This invention is directed to cyclic fluid separation processes andparticularly to cyclic adsorption processes having at least one vesselcontaining at least one adsorbent bed therein (herein an “adsorber bed”or “adsorber vessel”). Examples of commercial systems include, but arenot limited to, PSA and VPSA processes and systems wherein an adsorbentbed is used to separate a less absorbable component from a moreabsorbable component.

Conventional cyclic adsorption systems will typically have at least oneadsorbent bed that is cyclically pressurized by at least one positivedisplacement feed compressor and sometimes evacuated by at least onesuch vacuum compressor. In the present invention, one centrifugalmachine, designed for variable-speed operation, is used for bothpressurization and depressurization of a single adsorber bed andmultiple bed systems will have separate centrifugal machines; one forfeeding gas to the vessels and one for evacuating the vessels. Thesemachines offer improved process efficiencies through their reducedoperating power requirements when used with adsorber vessels that arelarge enough to result in process cycle times of not less than the ratioof Δ_(inertia) to 0.3 times the centrifugal compressor power that wouldbe consumed in the absence of inertial effects.

Centrifugal compressors driven by variable, high speed motors are knownand have been proposed for use in adsorption processes by the presentassignee as taught in U.S. Pat. No. 7,785,405B2 which is fullyincorporated by reference herein. Centrifugal compressors, sometimesreferred to as radial compressors, are considered a sub-class of dynamicaxisymmetric work-absorbing type turbo machinery. These machines arewell known although their application in cyclic adsorption process hasonly been proposed recently. Centrifugal machines typically operate athigh speeds and generate high pressure rises. The term “machines” isused herein to describe the operating machine which includes thecompressor/blower and the motor drive system.

The centrifugal compressors are preferably driven by direct drivevariable, high speed permanent magnet motors having variable frequencydrives which permit the compressor to cyclically accelerate from a lowoperating speed to a high operating speed and decelerate from highoperating speed to low operating speed at rapid rates as required bycurrent PSA or VPSA cycle times, such as less than a minute. Thesemotors are capable of operating the compressors at speeds greater than5000 RPM, preferably greater than 10,000 RPM, and most preferablygreater than 15,000 RPM. Preferably, for VPSA and PSA processes, the lowoperating speed will be not more than 7000 RPM and the high operatingspeed will be greater than 13,000 RPM.

As used herein, the term “centrifugal machines” is intended to includecentrifugal compressors driven by high speed induction motors, alsoknown as rotating transformers or asynchronous motors. An inductionmotor is a type of alternating current motor where power is supplied tothe rotor by means of electromagnetic induction. The electric motorturns because of the magnetic force exerted between the stationaryelectromagnet, the stator, and the rotating electromagnet, the rotor.

The compressors used in the inventive process can have single ormultiple stages, can have various impellers or blade configurations, canbe configured to operate in association with one or more beds and can beused in combination with other positive displacement machines. However,one compressor is typically used per service and, in one embodimentherein, the compressor is used in the absence of a positive displacementmachine. The centrifugal machine is selected such that its efficiency isat least 10% greater than the efficiency of a conventional positivedisplacement machine used in the same application.

FIG. 1 is a graph illustrating how the centrifugal compressor operatesat various operating speeds and pressures during a typical VPSA oxygenprocess. The curves demonstrate the operating conditions that willresult in surge, stonewall, and optimal efficiency. Again, the VPSAprocess is used only for illustrative purposes. As shown in FIG. 1, Line1 is the surge line, Line 2 is the best efficiency line, and Line 3 isthe stonewall line. The flow coefficient is 0.3 (the flow coefficient isa dimensionless number that can be used to calculate the mass flow) at aspeed that is 100% of the design speed and a pressure ratio across thecompressor of 1.65. If the compressor is maintained at 100% of designspeed and the pressure ratio across it suddenly drops to 1.6, then theflow coefficient will be 0.43 representing a 40% increase in flow. Forevery choice of operating speed there is a fixed relationship betweenthe pressure rise across the compressor and the flow that the compressorcan produce.

At the left of Line 1 in FIG. 1 is a condition known as “surge”. Thesurge point is defined herein as the minimum stable flow rate for agiven pressure rise across the centrifugal compressor. If the flow ratedecreases beyond this point, then the head pressure developed by thecompressor decreases causing a reverse pressure gradient at thecompressor discharge and a resulting backflow of gas. Once the pressurein the discharge line of the compressor drops below the pressuredeveloped by the impeller, the flow reverses once again. Thisalternating flow pattern has been found to be an unstable condition thatcan result in serious damage to the compressor impeller, drive mechanismand components. This condition must be avoided.

On the right of Line 3 is a condition known as “stonewall”. Stonewall isdefined herein as a condition where the compressor fails to develop anypressure head because the volumetric flow through the compressor is toohigh for a given rate of impeller rotation. Thus, a further reduction inpressure ratio does not result in additional flow. Although thiscondition will not damage the impeller, the power efficiency of thecompressor will be lowered. The occurrence of stonewall or a stonewallcondition should be minimized in order to ensure that it does not negatethe power benefits created by replacing the rotary lobe type blower withthe more efficient centrifugal compressor. Preferably, the cycle time isselected so that the compressor is in a stonewall condition for not morethan 40 percent, and preferably less than 30%, of the total cycle timeof the process.

As the pressure inside an adsorber vessel changes, the operating speedof the centrifugal compressor must be changed. The maximum efficiencycan be realized by maintaining the compressor slightly to the right (onFIG. 1) of its surge condition. For example and again referencing FIG.1, it is shown that at a pressure ratio across the compressor of 1.4,the compressor should be operated at 80% of its design speed. Generally,it is theoretically possible to control the speed of the compressor tomaintain optimal efficiency while avoiding surge and stonewallconditions. However, during points in time within the process when thepressure ratio suddenly drops, it is effectively impossible tocompletely avoid stonewall conditions. Although the compressor speed canbe suddenly increased by adding power to the compressor motor, the onlymeans of slowing the impeller down is to allow it to naturally coastdown as it does work on the gas or fluid that is being compressed.

The optimal efficiency line, shown in FIG. 1 as Line 2, is arelationship between operating speed and pressure ratio that maximizesthe efficiency of the machine. FIG. 2 shows how efficiency varies as afunction of pressure ratio and speed of operation. The graph shows thatfor each operating speed the efficiency is at a maximum value, close to85%, when the flow coefficient is approximately 0.3. The optimum flowwill vary with speed. At very low operating speed the optimal flowcoefficient is 0.28 while at very high operating speed the optimal flowcoefficient is 0.32. The optimal points on the graph of FIG. 2 translatedirectly into the optimal operating line shown in FIG. 1 as Line 2.

FIG. 3 is a graph showing the bed pressures inside the adsorber bed andthe speed of the centrifugal compressor driven by a direct drivevariable, high speed permanent magnet motor during a full cycle for anactual VPSA process for the separation of oxygen from air. This cycleincluded pressurization, depressurization, equalization, evacuation, andpurge steps. A single centrifugal compressor was used which produced aflow of 7000 ACFM (Actual Cubic Feet Per Minute) at an operating speedof 13,500 RPM. The graph in FIG. 3 contains a curve (represented by theline with triangles) showing how the adsorber vessel pressure changed asa function of time. Another curve (represented by the line withouttriangles) illustrates how the centrifugal machine speed varied withtime to match the changing pressure within the adsorber vessel. Theshaded areas (identified by arrows 1, 2 and 3) represent inertial energylosses of the compressor. The first shaded area (1) is during the periodof adsorber bed pressurization and the second shaded areas (2 and 3) areduring the period of adsorber bed depressurization. At the verybeginning of the cycle, the bed has just been purged and it is at a verylow pressure. The compressor speed is very high at this point since itis used to maintain the low pressure (high pressure ratio across thecompressor). A set of valves within the system is now switched so thatthe compressor can be used to feed atmospheric air to the adsorbervessel. The sudden change of pressure at the suction of the compressorfrom a vacuum to an ambient condition and change in pressure at thedischarge of the compressor from an ambient to a vacuum conditionresults in a sudden reduction in the pressure ratio across thecompressor (e.g. from 2 to 0.5) This change puts the compressor in thestonewall condition. As air flows through the compressor, it coasts downin speed and the adsorber bed gradually increases in pressure. As thecompressor coasts down in speed, the inertial energy of the impeller,motor rotor and shaft provide the energy needed to compress the gas intothe adsorber vessel. Stonewall is a very low efficiency conditionresulting in a low amount of the inertial energy, such as less than ½ inthe compressor used during this example, being used to compress gas withthe rest of the energy being wasted.

Once the compressor impeller sufficiently reduces speed to cause thesystem to exit the stonewall condition, energy is supplied to thecompressor to increase its speed to match the increasing pressure in theadsorber vessel. The speed reaches a peak point in the process as theadsorber vessel reaches the pressure at which the oxygen product gas canbe produced (or, for other processes, the less absorbable gas). Once theadsorber vessel becomes saturated with nitrogen (or the more absorbablegas), it is subsequently blown down. During this blow down step (2), thecentrifugal compressor is allowed to coast down in speed as it moves air(or other feed gas) from the feed side of the system through a vent asthe pressure in the adsorber vessel is relieved through another valve.The air is vented to the atmosphere or, in the case or other processes,otherwise captured as required. During this period of the cycle, 100% ofthe change in inertial energy of the impeller is lost without doing anyuseful work. It should be noted that the work is lost whether theprocess uses a centrifugal compressor or a conventional rotary lobeblower although the losses for the centrifugal compressor are higherbecause the flow of gas through the centrifugal compressor is muchhigher than through the rotary lobe blower.

After the adsorber vessel reaches atmospheric pressure, the compressoris used to pull the adsorber vessel down to a deep vacuum state. Atfirst the compressor is in a stonewall condition and continues to coastdown in speed as gas is removed from the vessel. Once the speed of thecompressor is low enough and the pressure ratio across it is highenough, the compressor comes out of it's stonewall condition. Power isnow supplied to the compressor to further evacuate the adsorber vessel.

As shown in the shaded areas of FIG. 3, a considerable amount of time isspent with the compressor in a stonewall condition and the power loss isappreciable. It was estimated that the wasted power corresponding to thelightly shaded portions of the cycle shown in FIGS. 3 (1 and 3)represent up to 15% of the total power consumption. Another 4.5% oftotal system power is wasted during the more darkly shaded portion (2)of the process shown in FIG. 3. Given that the switch from aconventional positive displacement type machine to a centrifugal machinecan save up to 30% of the total power consumption in a typical VPSAoxygen process (85% efficiency vs. 65% efficiency), it is imperative tominimize the inertial energy lost due to compressor stonewall tomaximize the power benefits received from the centrifugal machine.

The invention described herein involves selecting the adsorption processcycle, such as the VPSA process cycle as illustrated, in such a way thatthe inertial energy lost due to compressor stonewalling is smallrelative to the total amount of energy supplied to the system. This isdone by increasing the duration of the overall process cycle, which isaccomplished by increasing the size of the adsorber vessels relative tothe size of the centrifugal compressor(s). This is contrary to thecurrent direction of vessels and cycle design which has recently focusedon smaller beds and faster cycles. It has therefore been surprisinglyfound that cyclic adsorption systems using centrifugal compressors arenot more energy efficient than current systems using positivedisplacement type blowers without adjusting such systems to havesufficiently long process cycle times.

Although not wanting to be bound to theory, this requirement can beexplained by referring again to FIG. 3. It is believed that thislimitation in efficiency is shown where the moment of inertia of theimpeller and motor rotor is calculated to be 0.28 kg*² based on thematerials of construction and geometry of the centrifugal impeller andmotor rotor. The inertial effects are the calculated amount ofresistance to change in velocity which is caused by the spinning weight(or mass) of the impeller and motor rotor. This can be calculated by oneskilled in the art from the design and operating specifications of themachines.

At the start of feed it is known that the impeller will coast down inspeed from about 16,500 RPM to about 6600 RPM as can be seen in FIG. 3.During this time, the inertial energy of the compressor goes from 420 kJto 67 kJ for a total reduction in energy of 353 kJ. Similarly duringblowdown and the start of evacuation it is known that the impeller willcoast down in speed from about 13,500 RPM (the design speed referencedabove) to about 5,400 RPM. During this period, the inertial energy ofthe compressor goes from 277 to 44 kJ for a total change of 233 kJ. Asshown, the total change in inertial energy during the two coasting downperiods over a typical VPSA cycle is 586 kJ. This has now beendiscovered to be an important machinery design parameter that can becalculated for any selected motor plus centrifugal impeller (movingparts) and knowledge of the maximum and minimum speeds of operationduring cyclic operation. This parameter, change in inertia, is definedherein as Δ_(inertia) and represents the maximum possible energy thatcan be lost during a cycle due to inertia changes. Once the centrifugalcompressor and motor is selected, the inertial energy per cycle(Δ_(inertia)) is fixed and constant regardless of the duration of theoverall cycle. Although the above example demonstrates the calculationof Δ_(inertia) for one specific system, a general formulation can beused to determine Δ_(inertia) for any system. Such a formulation is wellknown in the field of Physics and is provided below:

KE _(max)=½·I·ω _(max) ²

KE _(min)=½·I·ω _(min) ²

Δ_(inertia) =KE _(max) −KE _(min)

In the above formulas, I is the moment of inertia of the rotor, shaft,and impeller around the axis of rotation, ω_(max) and ω_(min) are themaximum and minimum speeds of rotation of the rotor, shaft, and impellerexpressed in radians per unit time, and KE_(max) and KE_(min) are thekinetic energy of rotation of the rotor, shaft, and impeller.

Once Δ_(inertia) is known for a given compressor/motor design, it ispossible, with a knowledge of the particular process cycle time, tocalculate the power penalty associated with inertial energy losses. Thisis determined by dividing the inertial energy loss by the cycle time,represented by the formula:

$P_{inertia} = \frac{\Delta_{inertia}}{CT}$

In the above example for the VPSA process, the total inertial power lossis 17.7 kW (calculated as 586 kJ/cycle divided by a cycle time of 33.2seconds). In practice the actual inertial power loss will be slightlyless than this number because a small portion of the energy does usefulwork on the gas. Notwithstanding, the loss will be small enough not tohave a significant impact on the above expression.

The total power consumed by the centrifugal compressor will be thetheoretical power that would be consumed absent inertial effects+theinertial losses resulting from stonewall operation is represented by theformula:

$P_{cent} = {{P_{noinertia} + P_{inertia}} = {P_{noinertia} + \frac{\Delta_{inertia}}{CT}}}$

The value of P_(noinertia) will not depend upon the geometry of thecompressor or the selection of cycle time as is the case forP_(inertia). It will be a linear function of plant capacity. In theexample above, a commercial scale VPSA plant was operated to produce12.5 tons of oxygen per day. The measured power consumption was 90 kW.This was the total power consumption including inertial losses. Sincethe calculated inertial power loss was 17.7 kW, the above formulaprovides the power consumption of the centrifugal compressor absentinertial effects P_(noinertia)=72.3 kW. Expressing this as a function ofplant capacity results in a specific power consumption (absent inertialeffects) of P_(noinertia)=5.8 kW/TPD, a relationship that can be used tocalculate P_(noinertia) for any sized plant (multiply 5.8 kW by theplant design capacity in per tons per day (TPD)). Thus, P_(noinertia) isequal to 5.8 kW per (US short) TPD of plant capacity (907.2 kilogramsper day) which is a common production flow rate used in the industrialgas industry.

As was stated above, it is critical to select a cycle time of sufficientduration to ensure that the process using the centrifugal compressor isat least as energy efficient as the process using the conventionalblower (hereinafter a rotary lobe type blower), preferably moreefficient.

It is known that the power consumed by a process using a rotary lobeblower will be 1.3 times higher than a process using a centrifugalcompressor with no inertial effects. In other words, the 85% adiabaticefficiency for the centrifugal compressor is divided by the 65%adiabatic efficiency for the rotary lobe blower resulting in a powerratio of 1.3. Thus, to ensure that the power consumption when using acentrifugal compressor is less than what would result from using arotary lobe type blower, the following equation must hold true:

${1.3 \cdot P_{noinertia}} > {P_{noinertia} + \frac{\Delta_{inertia}}{CT}}$

Simplifying and rearranging for minimum cycle time yields:

${CT} > \frac{\Delta_{inertia}}{0.3 \cdot P_{noinertia}}$

In this example, the selection of compressor resulted in a Δ_(inertia)of 586 kJ/cycle. The centrifugal compressor power, absent inertialeffects, was 72.3.

Referring again to the equations, the minimum cycle time resulting in apower benefit using a centrifugal compressor in place of a rotary lobetype blower in this example is 27 seconds. It can now be determined thatany cycle time less than this value results in power consumption that ishigher than for a rotary lobe type blower in this process. Preferably,when used in a cyclic adsorption process, the total power required forthe centrifugal machine is selected to be at least 10% less than thepower required for a positive displacement machine when used in the sameprocess.

Because the above example is fairly typical of a VPSA process design andusing the best variable speed centrifugal compressor design adapted forthis VPSA process, it can be concluded that a cycle time of at least 27seconds is required in order to ensure that the use of a centrifugalcompressor results in a net energy savings relative to a conventionalrotary lobe type blower.

While the above examples were calculated using a typical VPSA processfor the separation of oxygen from air, it is believed that these samefindings can be employed for any cyclic adsorption process using one ormore centrifugal compressors for rapid pressurizing and/ordepressurizing. This includes adsorption processes using multipleadsorber beds and having any bed or vessel configuration such as radialor axial configurations.

It should be apparent to those skilled in the art that the subjectinvention is not limited by the examples provided herein which have beenprovided to merely demonstrate the operability of the present invention.The selection of adsorption process, process conditions, cycle times,and adsorber vessel size can be determined by one skilled in the artfrom the specification without departing from the spirit of theinvention as herein disclosed and described. The scope of this inventionincludes equivalent embodiments, modifications, and variations that fallwithin the scope of the attached claims.

What is claimed is:
 1. A cyclic adsorption system having at least onevessel containing at least one adsorbent bed therein for receiving afeed gas at a pressure wherein during a cycle at least one more readilyadsorbed gas in the feed gas is adsorbed by the adsorbent as the feedgas is passed across the bed producing a product gas rich in at leastone less readily adsorbed gas and evacuating the at least one vessel toa lower pressure whereby the at least one more readily adsorbed gas isdesorbed from the adsorbent bed, wherein the cycle is driven by one ormore variable speed centrifugal machines driven by a direct drivevariable high-speed permanent magnet motor having a variable frequencydrive and designed to accelerate from a low operating speed to a highoperating speed to obtain a cycle pressure condition and then todecelerate from high operating speed to a low operating speed to obtaina lower cycle pressure condition and the cycle time is controlled suchthat the one or more centrifugal machines is in a stonewall conditionfor a period of not more than 40 percent of the cycle time under theconditions employed.
 2. The system of claim 1 wherein the system isdesigned to accelerate from a low operating speed to a high operatingspeed to obtain a high cycle pressure condition and to decelerate fromhigh operating speed to a low operating speed to obtain a low cyclepressure condition.
 3. The system of claim 2 wherein the high pressurecondition is 19 to 24 psia and the low pressure condition is 6 to 8psia.
 4. The system of claim 3 wherein the adsorption system is designedto conduct a VPSA or PSA process.
 5. The system of claim 4 wherein theVPSA or PSA process is for the production of oxygen from air.
 6. Thesystem of claim 1 wherein the one or more centrifugal machines is in astonewall condition for a period of less than 30% of the cycle time. 7.The system of claim 1 wherein the process cycle time of at least 27seconds.
 8. A system designed for conducting an adsorption process forseparating a more absorbable component gas from a less absorbablecomponent gas comprising at least one variable speed centrifugalcompressor being in fluid communication with at least one adsorbervessel and capable of accelerating from a low operating speed to a highoperating speed and decelerating from high operating speed to lowoperating speed during the process cycle wherein the cycle time iscontrolled such that the centrifugal compressor is in a stonewallcondition for a period of not more than 40 percent of the cycle timeunder the conditions employed.
 9. The system of claim 8 wherein thecentrifugal compressor is driven by a direct drive variable high-speedpermanent magnet motor having a variable frequency drive.
 10. The systemof claim 8 wherein the accelerating from a low operating speed to a highoperating speed obtains a first cycle pressure condition and thedecelerating from high operating speed to a low operating speed obtainsa lower cycle pressure condition.
 11. The system of claim 10 wherein thefirst pressure condition is 19 to 24 psia and the lower pressurecondition is 6 to 8 psia.
 12. The system of claim 8 wherein the size ofthe adsorber vessels relative to the size of the centrifugal compressoris adjusted to support a cycle time of sufficient duration to ensurethat the process using the centrifugal compressor is at least as energyefficient as the process using the conventional blower.
 13. The systemof claim 12 wherein the process cycle time of at least 27 seconds.